There are a number of control signals that are typically used to control synchronous dynamic random access memory (SDRAM). These control signals include a clock signal and clock enable signal (or simply CKE). In general, the commands used in SDRAM are timed relative to one or more edges of the clock signal. The CKE refers to a signal that enables or disables the clock signal. When the CKE is asserted an SDRAM responds to commands relative to one or more edges of the clock signal. When the CKE is de-asserted the SDRAM behaves as if the clock has stopped.
A chip select signal refers to another control signal frequently used in SDRAM systems. The chip select signal is used to select or deselect SDRAM devices. The term “rank” refers to a group of SDRAM devices that are tied to a particular chip select signal.